Method for converting uranium ammonium phosphate to uranyl fluoride



United States Patent METHOD FOR CONVERTING URANIUM AMMO- NIUM PHOSPHATE TO URANYL FLUORIDE Seymour Bernstein and Harvey A. Bernhardt, Oak Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application April 25, 1950, Serial No. 158,062

Claims. (Cl. 23-145) Our invention relates to a method of converting uranyl ammonium phosphate to other uranium values and more particularly to the conversion of uranyl ammonium phosphate to UOzFz.

It has been discovered that it is advantageous to recover uranium from certain waste solutions in the form of various uranyl phosphate precipitates, such as uranyl hydrogen phosphate, uranyl sodium phosphate, and particularly uranyl ammonium phosphate. However, it has been found extremely difficult to treat these precipitates in order to economically convert the uranium contained therein into a form, such as UFa, which is readily returnable to various plant processes. Attempts to fluorinate these uranyl phosphates directly to UFs have resulted in the formation of by-product phosphorous fluorides which consume costly elemental fluorine and are difficult to separate from the UFB. Elforts have therefore been made to produce UOzFz as an intermediate compound since it is known that UOzFa may be feasibly converted to UFs by reaction with fluorine. Hydrofluorination of uranyl ammonium phosphate, however, proved unsatisfactory for a commercial process since it resulted in the formation of the corrosive by-product ammonium hydrogen fluoride (NHaRHF) which is a solid at room temperature and which plugs process exhaust lines and valves.

An object of our present invention, therefore, is to provide a process for converting uranyl ammonium phosphate into another uranium compound which is easily and economically convertible to UFa.

Another object of our invention is to provide an improved method for converting uranyl ammonium phosphate to UOzFa.

A further object is to provide a process for substantially quantitatively converting uranyl ammonium phosphate to UOzFz without the formation of corrosive byproducts which are solids at room temperature.

Additional objects and advantages of our invention will be apparent from the following description.

In accordance with our present invention, the substantially complete conversion of uranyl ammonium phosphate to UOzFz may be effected in an economically feasible manner by heating the uranyl ammonium phosphate within a narrow temperature range until substantially all the uranyl ammonium phosphate is converted to uranyl hydrogen phosphate, and then contacting the resulting uranyl hydrogen phosphate with anhydrous HF at a suitably elevated temperature until substantially all the uranyl hydrogen phosphate is converted to UOzFz.

Using this two step method, we avoid the formation of the extremely undesirable NH4F.HF mentioned above and also obtain a product which is very reactive with many reagents but which we find is particularly suitable for reaction with fluorine to form UFs without the expenditure of the economically unfeasible amount of fluorine which would be required to directly fluorinate any of the uranyl phosphates described above.

In general, any form of uranyl ammonium phosphate may be eifectively decomposed in the preliminary step "ice of our process to form uranyl hydrogen phosphate. However, the uranyl ammonium phosphate which is commonly recovered from waste solutions is UOzNHrPO-sAHaO and we have discovered that unexpected advantages result from drying this UOzNHrPOsAl-IzO to and utilizing the latter for decomposition to uranyl hydrogen phosphate. We find that this drying may be easily accomplished, for example, by heating the UOzNHaPOrAHaO at a temperature of approximately 50 C. to approximately C. for approximately 6 hours to approximately 24 hours at atmospheric pressure. However, a temperature of about 60 C. is generally preferred.

The resulting UO2NH4PO4.2H2O is much easier to handle than the original UO2NH4PO4.4H2O and there is a marked improvement in the absence of NH3 from the uranyl hydrogen phosphate product of the following decomposition step.

The temperature for the conversion of uranyl ammonium phosphate to uranyl hydrogen phosphate is somewhat critical, for a greatly increased rate of formation of uranyl hydrogen phosphate appears to fall within the region of approximately 450 C. Uranium phosphate compounds other than uranyl hydrogen phosphate tend to be formed in progressively larger amounts as the temperature is increased above 450 C., for the same reaction period. During the following hydrofluorination step, these phosphates would form uranium compounds other than the desired UO2F2. At temperatures substantially lower than 450 C., the decomposition reaction becomes uneconomically slow. The decomposition should therefore be effected Within the range of 300 C.500 C., and preferably at approximately 450 C.

We find that the time of the decomposition reaction is important in obtaining a high purity product uranyl hydrogen phosphate. Although a two hour period provides satisfactory results, and complete removal of NHs, a period of four to five hours appears to be best.

During the decomposition reaction, it is desirable to sweep out gaseous decomposition products by means of an inert gas. Although, in general, any inert gas may be used for this purpose, air has been found to be completely satisfactory and is obviously to be preferred for reasons of economy. The quantity of gas flow is not critical but insufficient flow retards the decomposition reaction. In general, we find that a gas velocity of approximately 2.7 cm./sec. to approximately 4.1 cm./sec. per 10 grams of starting material removes the decomposition products approximately as fast as they are formed when the reaction conditions described above are utilized.

In the final step of our invention, the temperature for the hydrofluorination of uranyl hydrogen phosphate to U02Fz is not as critical as the temperature for the preceding decomposition step. However, we find that a range of approximately 400 C. to approximately 500 C. is particularly advantageous, and the optimum temperature appears to be about 500 C.

An anhydrous HF gas velocity of approximately 0.82 cm./sec. to approximately 1.1 cm./sec. per 10 grams of uranyl hydrogen phosphate effects substantially complete conversion to UOzFz in an optimum time. although the higher part of this range appears to cause the reaction to proceed somewhat faster without deleterious side effects.

The time of the reaction is not critical but depends upon the particular combination of quantities and reaction conditions described above. For example, if a gas velocity of 1.1 cm./sec. of anhydrous HF per 10 grams uranyl hydrogen phosphate is utilized at a temperature of about 500' C., approximately 3 hours appear to provide optimum results. If a gas velocity of 0.82 cm./sec. is utilized, approximately 4 hours appear to be best.

In a preferred procedure for carrying out the two step conversion of uranyl ammonium phosphate to UOzFz in accordance with our invention, UOzNH4PO4.2HzO is placed in HF resistant reaction trays in an HF resistant reactor, which is then inserted in a furnace. The air flow is started through the reactor, as the furnace is turned on, at a velocity of approximately 4.1 cm./sec. per 10 grams starting material. The reaction zone is heated to a temperature of about 450 C. and maintained there for approximately four hours. The air flow is then cut off, the temperature raised to approximately 500 C., and anhydrous HF gas, at a velocity of approximately 0.82 cm./ sec. per 10 grams starting material, is passed through the reaction zone for about four hours. The material remaining in the reaction trays is the UOzFz product.

After treating uranyl ammonium phosphate in accordance with the above procedure, we find that the UOzFz product is approximately ninety-eight percent pure and may be readily and economically fluorinated to form UFs of sufficient purity for practically any desired use.

The following specific example illustrates our invention in greater detail.

Example I A charge of 11 grams of UOZNH4PO4.4H2O was dried at 60 C. for 16 hours. The resulting UO2NH4PO42H2O was placed in a nickel tray which was loaded into a nickel tube reactor surrounded by an electrically heated tube furnace. The furnace was then turned on and the reaction zone was heated to a temperature of approximately 450 C. When this temperature was reached, an air flow of 1% liters per minute was passed through the reactor and maintained for four hours throughout which period the temperature was held at 450 C. The air was then turned off, the temperature raised to 500 C., and an anhydrous HF gas flow of 300 cc. per minute was passed through the reaction zone for four hours. The furnace was then turned off, the reaction zone permitted to cool, and the substantially pure UO2F2 product removed.

Analysis indicated that the intermediate decomposition compound produced in the above example contained 64.5% U, 26.6% P04 and 0.09% NH4+. The theoretical analysis of uranyl hydrogen phosphate is 65.0% U and 26.0% PO4 Thus, substantially complete conversion of uranyl ammonium phosphate to uranyl hydrogen phosphate had been accomplished.

Analysis also indicated that the final hydrofluorination product assayed at 77.0% U (1.34% U), 12.8% F, 0.43% P04 and no detachtable NH4. The theoretical analysis of UOzFz is 77.3% U and 12.3% F. Thus, the final product was approximately 98% UOzFz.

In general, it may be said that the above example is merely illustrative and should not be construed as limiting the scope of our invention which should be understood to be limited only as indicated by the appended claims.

What is claimed is:

1. A method of converting uranyl ammonium phoshate to uranyl fluoride which comprises heating the uranyl ammonium phosphate at a temperature of 300- 500 C. until substantially all the uranyl ammonium phosphate is converted to uranyl hydrogen phosphate and contacting said uranyl hydrogen phosphate with anhydrous HF gas at a temperature of 350 to 600 C. until said uranyl hydrogen phosphate is substantially completely converted to UOzFz.

2. A method of converting UO2NH4PO4.4H2O to UO2F2 which comprises drying the UO2NH4PO4.4H2O at a temperature of approximately 60 C. until said UO2NH4PO4.4H2O is substantially all dehydrated to UO2NH4PO42H2O, heating said UO2NH4PO4.2H2O at a temperature of 300500 C. until substantially all the UO2NH4PO4.2H2O is decomposed to uranyl hydrogen phosphate and contacting said uranyl hydrogen phosphate with anhydrous HF gas at a temperature of 350- 600 C. until said uranyl hydrogen phosphate is substantially all converted to UO2F2.

3. A method of converting UO2NH4PO42H2O to uranyl fluoride which comprises heating the at a temperature of 300-500 C. until substantially all the UOZNH-rPOrZI-IQO is converted to uranyl hydrogen phosphate and contacting said uranyl hydrogen phosphate with anhydrous HF gas at a temperature of 350 to 600 C. until said uranyl hydrogen phosphate is substantially completely converted to UOzFs.

4. A method of forming substantially pure uranyl hydrogen phosphate from uranyl ammonium phosphate which comprises heating the uranyl ammonium phosphate at a temperature of 300-500 C. until the decomposition of said uranyl ammonium phosphate to uranyl hydrogen phosphate is substantially complete.

5. A method of converting UO2NH4PO4.4H2O to uranyl hydrogen phosphate which comprises drying the UO2NH4PO4.4H2O at a temperature of approximately 60 C. until said UO2NH4PO4.4H2O is substantially all dehydrated to UO2NH4PO42H2O and heating said from UO2NH4PO4AH2O which comprises heating the UO2NH4PO4.4H2O at a temperature of approximately 60 C. and in contact with the ambient atmosphere until said UO2NH4PO4AH2O is dehydrated to 9. A process for converting UO2NH4PO4.2H2O to substantially pure UOzFz which comprises heating said UO2NH4PO4.2H2O at a temperature of approximately 450 C. for 2-5 hours, and contacting the resulting uranyl hydrogen phosphate with anhydrous I-IF gas at a temperature of 350-600 C. for 2-4 hours.

10. A method of forming substantially pure from UO2NH4PO4.4HO2 which comprises heating the UO2NH4PO4AH2O at a temperature of 50 C. C. and in contact with the ambient atmosphere until said UO2NH4PO4.4H2O is dehydrated to UO2NH4PO4.2H2O.

References Cited in the file of this patent Sutton: Chemical News, volume 1, pages 97-8 (1860).

Kitchin: Chemical News, volume 27, page 199 (1873).

Friend: Textbook of Inorganic Chemistry, volume 7, part 3, page 330 (1926). Published by Charles Griffin and Company, London. 

2. A METHOD OF VONVERTING UO2NH4PO4.5H2O TO UO2F2 WHICH COMPRISES DRYING THE UO2NH4PO4.4H2O AT TEMPERATURE OF APPROXIMATELY 60*C. UNTIL SAID UO2NH4PO4.4H2O IS SUBSTANTIALLY ALL DEGYDRATED TO UO2NH4PO4.2H2O, HEATING SAID UO2NH4PO4.2H2O AT A TEMPERATURE OF 300-500*C. UNTIL SUBSTANTIALLT ALL THE UO2NH4PO4.2H2O IS DECOMPOSED TO URANYL HYDROGEN PHOSPHATE AND CONTACTING SAID URANYL HYDROGEN PHOSPHATE WITH ANHYDROUS HF GAS AT A TEMPERATURE OF 350600*C. UNTIL SAID URANYL HYDROFGEN PHOSPHATE IS SUBSTANTIALLY ALL COVERTED TO UO2F2. 